Most hydrophones are made from piezoelectric transducers that transform pressure waves impinging on the instrument into a voltage—this voltage, if calibrated correctly, gives a precise reading of pressure wave amplitude. Unfortunately a hydrophone does not produce the same output for a given acoustic amplitude at all acoustic frequencies. Therefore calibration data must be measured at many frequencies spanning the frequency range of the hydrophone.
Hydrophones play a large role for scientists and oceanographers in ocean research. The ability to monitor underwater acoustics allows for applications such as tracking whale activity, military purposes, and at very low frequencies, the assessment of earthquakes, underwater landslides, and turbidity currents. Digital hydrophones have now been developed to reach frequencies as low as 0.001 Hz. Presently hydrophone calibration facilities calibrate only the analog portion of a hydrophone system, and full end to end digital calibrations are not provided. This poses a problem for assessing the full system accuracy of both amplitude sensitivity and phase delays. It is desirable to measure both the sensitivity and phase delay at the temperatures and pressures that the hydrophone will be used at since the hydrophone sensitivity changes with both temperature and pressure. Low frequency (<15Hz) calibration systems are not available. A previously proposed system (U.S. Pat. No. 3,352,144 A) used cable gland seals that are unsuitable for high pressure use, and an insonification drive mechanism that is both noisy and limited to low static pressures.